With the recent downsizing of electrical and electronic devices, inductor components including magnetic materials are also demanded to be smaller and more efficient. For example, a choke coil, which is an inductor component used in a high-frequency circuit, includes either a ferrite magnetic core made of ferrite powder or a composite magnetic material (a compressed powder magnetic core). The composite magnetic material is a compact of metal magnetic powder.
The ferrite magnetic core has disadvantages of low saturation magnetic flux density and low DC superimposing characteristics. Therefore, in order to ensure sufficient DC superimposing characteristics, conventional ferrite magnetic cores are provided with a gap of several hundreds of micrometers in a direction perpendicular to the magnetic path, thereby keeping the inductance L at DC superimposition. However, such a large gap causes a beat note, and also a leakage magnetic flux particularly in high-frequency ranges, thereby causing serious copper loss in the copper windings.
In contrast, the composite magnetic material, which is manufactured by molding metal magnetic powder, is advantageous for use in small devices because its saturation magnetic flux density is far greater than that of the ferrite magnetic core. Unlike the ferrite magnetic core, the composite magnetic material can be used without forming a gap, thereby having small beat note and low copper loss caused by the leakage magnetic flux.
The composite magnetic material, however, cannot be said to be superior to the ferrite magnetic core in terms of magnetic permeability and core loss. In particular, when used in a choke coil or an inductor, the composite magnetic material has large core loss, and hence, the core is likely to rise in temperature. For this reason, it is difficult to downsize inductor components containing the composite magnetic material. Furthermore, the composite magnetic material must have a large mold density in order to have high magnetic properties. The molding pressure required is not less than 6 ton/cm2, or is not less than 10 ton/cm2 depending on the product.
The core loss of a composite magnetic material is usually composed of an eddy current loss and a hysteresis loss. In general, metal magnetic powder has low intrinsic resistivity. Therefore, if the magnetic field changes, an eddy current flows so as to reduce this change, thus raising the problem of eddy current loss. The eddy current loss increases as the square of the frequency and the square of the area where the eddy current flows. The area where the eddy current flows can be reduced from the entire core containing the metal magnetic particles to only within the metal magnetic particles by coating the surface of the metal magnetic particles composing the metal magnetic powder with an insulating material. As a result, the eddy current loss can be reduced.
In addition, as the composite magnetic material is molded under high pressure, a large number of process strains are introduced into the compact. The composite magnetic material is thus decreased in the magnetic permeability and is increased in the hysteresis loss. To avoid this problem, after being molded, the compact is heat-treated to relax the strains, if necessary. In general, the relaxation of the strains introduced into the metal magnetic powder occurs at a heat-treatment temperature that is at least half the melting point. In order to sufficiently relax the strains in Fe-rich alloy, the compact must be heat-treated at 600° C. or more, and preferably at 700° C. or more. In other words, in the case of using the composite magnetic material, it is essential to heat-treat the compact at a high temperature while the insulation between the metal magnetic particles is maintained.
Examples of the insulating binder used in the composite magnetic material include epoxy resin, phenol resin, and vinyl chloride resin. These organic resins, however, have low heat resistance and are thermally decomposed if the compact is heat-treated at high temperature to relax the strains. For this reason, these insulating binders cannot be used.
To overcome this problem, the use of polysiloxane resin has been proposed (PLT 1, for example).